Lateral double diffused insulated gate field effect transistors (sometimes known as LDMOS transistors) are the power devices of choice for integration in very large scale integrated circuit (VLSI) logic processes. The reduced surface field (RESURF) design technique provides better tradeoff between breakdown voltage and specific on-resistance (r.sub.ds (on)) when compared to conventional LDMOS device designs. A RESURF n-channel LDMOS device will have an (N) drift region that surrounds an (N+) drain, given a (P-) semiconductor substrate. Relatively thick LOCOS oxide is grown on a portion of the drift region. A relatively deep (P) implant is used to make a body or (P) well of an insulated gate field effect transistor (IGFET) which spaces the drift region from an (N+) source region which is formed within the body. A (P+) back gate connection is also formed within the IGFET body implant region. A conductive gate is formed over and insulated from the IGFET body to extend from the source region over the body to the lateral margin of the LOCOS oxide and preferably extends onto a portion of this thicker oxide.
For a high voltage power device, the r.sub.ds (on) should be relatively low. In order to obtain a low on-resistance, the dopant in the drift region must be of a relatively high concentration. However, an epitaxial layer with such a high concentration will make it very difficult to diffuse the IGFET body having a comparable doping concentration and will make it difficult to control the breakdown voltage of the RESURF IGFET. Furthermore, such a diffusion process would be incompatible with existing CMOS processes.
From the foregoing, it may be appreciated that a need has arisen to form a drift region with a high doping concentration and still be able to diffuse a comparably concentrated IGFET body in order to develop a RESURF LDMOS transistor having a low on-resistance and controllable breakdown voltage.